This disclosure relates generally to the field of electrically operated “pulsed” neutron sources or “generators” used for evaluating neutron interaction properties of subsurface formations from within wellbores. More specifically, the disclosure relates to structures and operating methods for such pulsed neutron generators in which a heated electron emitting (“dispenser”) cathode may have longer lifetime than such pulsed neutron generators known in the art that use cathodes sensitive to degradation, such as dispenser cathodes. or cold cathodes, such as nanotipped emitters.
Pulsed neutron generators (PNGs) are known in the art for use in wellbore formation evaluation (“logging”) tools for evaluating neutron interaction properties of formations penetrated by a wellbore. Porosity, salinity, formation density, hydrogen content, formation elemental fractions, etc., can all be determined by measurements of interaction products of high energy neutrons from a PNG with such formations. Such well logging tools may include a sealed, hydrogen isotope fusion reaction tube to generate controlled duration pulses or “bursts” of neutrons at a specific energy level (usually 14 million electron volts—“MeV” for, example, for a deuterium-tritium fusion reaction). An important component of a PNG is an ion generator (“ionizer”) disposed within the sealed fusion reaction tube that generates hydrogen isotope ions. The hydrogen isotope ions are accelerated in an accelerator section of the sealed fusion reaction tube to produce nuclear fusion reactions in a metal-hydride target containing high concentrations of adsorbed hydrogen isotope atoms. The ions are generated in the ion generator by causing a molecule or atom of gas to be impacted by a sufficiently energetic electron and thereby stripping a bound electron from the molecule or atom. The energy of the incident (impacting) electron can vary from a few tens of electron volts (eV) to a few hundred eV. In “hot” cathode-based neutron tubes, the electrons are produced by a thermionic material(s), e.g., barium oxide, strontium oxide and calcium oxide imbedded onto an electrically heated cathode body. Such type of cathode is typically biased at or near (e.g., within a few volts) ground potential. Such cathode material needs to be heated to high temperature, typically about 1000° C., and a suitable electric field needs to be applied proximate the surface of the cathode material to extract and accelerate/energize the thermionic electrons. Such cathodes are referred to as “dispenser” cathodes. A dispenser cathode emits more electrons as its temperature rises, but in order to provide sufficient ionization energy to the thermionic electrons emitted from the cathode, a high transmissivity (at least 75%) grid or an electrode may be positioned near the face of the cathode and is biased positive at a potential with reference to the cathode from a few tens to a few hundreds of volts (the grid voltage referred herein as Vgrid). Eventually the grid, and/or other metal surfaces biased to Vgrid, collect emitted electrons, constituting a grid current, referred to herein as Igrid. The cathode temperature, and corresponding electron emission rate, may be controlled such that Igrid reaches a value that causes sufficient ion formation for desired neutron production
In order to maintain a stable neutron output, several voltage and/or current control loops may be used in connection with a typical neutron tube. One of these control loops maintains a constant Igrid at a constant Vgrid by adjusting the cathode heater current (Icat), and thereby the cathode temperature, via a negative feedback loop; that is, if Igrid rises above a set point then Icat is lowered (so that thermionic electron emission is correspondingly reduced by reduction in cathode temperature); conversely, if Igrid falls below its set point then Icat is raised to that Igrid increases to the set point.
Over time, as the neutron tube is used, the cathode may begin to degrade. As a result, Icat (and the corresponding cathode temperature) needs to be gradually increased to maintain the set point Igrid. The lifetime of dispenser cathodes is nominally rated to be over 10,000 hours by their manufacturers, but such lifetime rating is relevant only when the cathode is operated in a high quality vacuum. For neutron tubes used in well logging instruments, however, some cathodes have been known to last only a few hundred hours. Even if such degraded cathodes can still emit electrons, their end of life is generally defined as the point in time when the cathode can no longer maintain a selected Igrid at a maximum acceptable Icat, such maximum current being related, for example, to available power to operate the cathode and self-destructive limits on the cathode current.
Neutron tube dispenser cathode failures were reproduced in the laboratory, while operating parameters were recorded in order to establish how cathode degradation over time proceeds to eventual cathode failure. FIG. 1 shows an example of cathode current needed to maintain a selected setpoint grid current with respect to time. As shown in FIG. 1, neutron tube dispenser (hot) cathode degradation may be characterized, with reference to the cathode current, Icat, as gradual at first, but then suddenly becomes very rapid. The time at which the rapid degradation and ultimate cathode failure takes place in neutron generators is substantially shorter than advertised/expected lifetimes of commercially available cathodes, and more significantly, the rapid degradation may take place at unpredictable times.
What is needed is a neutron tube structure and operating technique that can reduce the effects of sudden cathode degradation in order to extend the useful lifetime thereof.